1. Field of the Invention
This invention relates to a process for the production of acetic acid and, in particular, to an improved process for the reduction and/or removal of permanganate reducing compounds formed by the carbonylation of methanol in the presence of a Group VIII metal carbonylation catalyst to produce acetic acid. More specifically, this invention relates to an improved process for reducing and/or removing permanganate reducing compounds or their precursors from intermediate streams during the formation of acetic acid by said carbonylation processes.
2. Technical Background
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, the nature of the halide promoter is not generally critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. No. 5,001,259, issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25, 1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; and European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e., 4 weight percent or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14-15 wt. % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, a specified concentration of iodide ions over and above the iodide ion that is present as hydrogen iodide. This iodide ion is a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as “a finite concentration” of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide, which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are herein incorporated by reference.
It has been found that although a low water carbonylation process for producing acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid, the amount of other impurities, present generally in trace amounts, can be increased by a low water carbonylation process, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions.
These trace impurities affect quality of acetic acid, especially when they are recirculated through the reaction process, which, among other things, can result in the build up over time of these impurities. The impurities that decrease the permanganate time of the acetic acid, a quality test commonly used in the acetic acid industry, include carbonyl compounds and unsaturated carbonyl compounds. As used herein, the phrase “carbonyl” is intended to mean compounds that contain aldehyde or ketone functional groups, which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction, 75, 369-380 (1998), for further discussion on impurities in a carbonylation process.
The present invention is directed to reducing and/or removing permanganate reducing compounds (PRC's) such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof. The present invention may also lead to reduction of propionic acid.
The carbonyl impurities described above, such as acetaldehyde, may react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirable to remove multi-carbon alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to poison the catalyst used in the production of vinyl acetate, a product commonly produced from acetic acid. Thus, the present invention may also lead to reduction or removal of multi-carbon alkyl iodides, in particular C2-12 alkyl iodide compounds. Accordingly, because many impurities originate with acetaldehyde, it is a primary objective to remove carbonyl impurities, notably acetaldehyde, from the process so as to reduce the multi-carbon alkyl iodide content.
Conventional techniques to remove such impurities include treating the acetic acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the low-water carbonylation process and purification treatment described above frequently remains somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Because a sufficient permanganate time is an important commercial test, which the acid product may be required to meet to be suitable for many uses, the presence of impurities that decrease permanganate time is objectionable. Moreover, it has not been economically or commercially feasible to remove minute quantities of these impurities from the acetic acid by distillation because some of the impurities have boiling points close to that of the acetic acid product or halogen-containing catalyst promoters, such as methyl iodide.
It has thus become important to identify economically viable methods of removing impurities elsewhere in the carbonylation process without contaminating the final product or adding unnecessary costs. For example, a method for manufacturing high purity acetic acid by adjusting the acetaldehyde concentration of the reaction solution below a certain amount, such as 1500 ppm, has been disclosed. It is stated that by maintaining the acetaldehyde concentration below this threshold, it is possible to suppress the formation of impurities such that one need only distill the crude acetic acid product to obtain high purity acetic acid.
The art has also disclosed that carbonyl impurities present in the acetic acid product streams generally concentrate in the overhead from the light ends column. Accordingly, the light ends column overhead has been treated with an amine compound (such as hydroxylamine), which reacts with the carbonyl compounds to form oxime derivatives that can be separated from the remaining overhead by distillation, resulting in an acetic acid product with improved permanganate time.
Other processes have been described for producing high purity acetic acid in which it is stated that an acetaldehyde concentration of 400 ppm or less is maintained in the reactor by using distillation to remove acetaldehyde. Streams suggested for processing to remove acetaldehyde include a light phase containing primarily water, acetic acid and methyl acetate; a heavy phase containing primarily methyl iodide, methyl acetate and acetic acid; an overhead stream containing primarily methyl iodide and methyl acetate; or a recirculating stream formed by combining the light and heavy phase.
It has been disclosed in commonly assigned U.S. Pat. Nos. 6,143,930 and 6,339,171 that it is possible to significantly reduce the undesirable impurities in the acetic acid product by performing a multi-stage purification on the light ends column overhead. These patents disclose a purification process in which the light ends overhead is distilled twice, in each case taking the acetaldehyde overhead and returning a methyl iodide rich residuum to the reactor. The acetaldehyde-rich distillate obtained after the two distillation steps is optionally extracted with water to remove the majority of the acetaldehyde for disposal, leaving a significantly lower acetaldehyde concentration in the raffinate that is recycled to the reactor. U.S. Pat. Nos. 6,143,930 and 6,339,171 are incorporated herein by reference in their entirety.
While the above-described processes have been successful in removing carbonyl impurities from the carbonylation system and for the most part controlling acetaldehyde levels and permanganate time problems in the final acetic acid product, further improvements can still be made. Accordingly, there remains a need for alternative processes to improve the efficiency of acetaldehyde removal. The present invention provides one such alternative solution.